Pore Scale Modelling of Porous Layers in a PEFC.


Jürgen Becker, Andreas Wiegmann
Fraunhofer ITWM, Kaiserslautern, Germany


Abstract 

The membrane electrode assembly (MEA) of a PEFC consists of several porous layers on
both the cathode and the anode side: the gas diffusion layer (GDL), the catalyst layer (CL)
and sometimes a micro-porous layer (MPL) in between. The physical properties of such a
porous layer are described by its macro-homogeneous properties, e.g. porosity, diffusivity,
permeability and conductivity.
Improving the performance of the cell is possible by improving the cell as a whole, and also
by optimizing each of the layers to its requirements. On the one hand, one should choose
the material with the best chemical properties. On the other hand, as material connectivity
and pore morphology have a major impact on the properties of porous media, improving the
micro-structure of each layer is equally important. Finding the best micro-structure
experimentally is often too costly or not possible, because any change would require
changes in the production process. Computer simulations help to determine the effective
material properties of a layer without the need to manufacture the layer first.
Therefore, in this talk we will present
1. how to create 3D virtual structure models of each layer,
2. how to determine the effective properties of the material, and
3. how to validate this approach.
The 3D structure models are created using stochastic processes; For example, GDL models
are created by placing fibres randomly using an anisotropic direction distribution. These
fibres may be straight or bent, where the bending is determined by its own random process.
For MPL and CL, agglomerate structures of carbon particles are created randomly.
Using these 3D models, the effective properties of the layer can be predicted. This requires
solving the appropriate partial differential equations on the micro-structural model; For
example, the permeability can be obtained from the solution of the Stokes equation.
To validate this approach we compare experimentally measured GDL properties with
numerically determined properties. For this purpose, a tomography image of the GDL is
used as model to ensure micro-structural geometric similarity between experiment and
simulation.


Math2Market GmbH
Last modified: Fri Oct 22 09:56:15 W. Europe Standard 2010